1. Field
The present invention generally relates to wireless communication systems and more particularly to a method and system for site selection transmit diversity (SSTD) in a CDMA data communication system.
2. Related Art
The current generation of cellular phone systems offers more services than those of previous generations, such as data services. First and second generation cellular communication systems were typically used mostly for voice services. Second generation systems began adding limited data services, albeit at low data rates. Third generation systems, such as the Code Division Multiple Access (“CDMA”) High Data Rate (“HDR”) system, offer integrated data capabilities with much higher data rates than that of second generation systems, which are capable of offering services such as streamed audio and video.
A cellular network consists of many geographic cells, each of which may contain multiple sectors. Inside each cell, there is a base station. A user typically communicates with the network through the sector that provides the best signal. When a mobile user changes location, the user may communicate with the network through a different sector that provides the most reliable signal. Techniques for handoff in second generation CDMA communication system are known in the art. However, CDMA data communication systems, such as CDMA HDR, present new problems when a mobile unit selects a new sector.
One such problem occurs when a user switches among sectors too quickly. In a conventional CDMA cellular system, data traffic, which includes voice, is routed to each sector that is actively communicating with a mobile unit, possibly using multiple base stations. Consequently, all active sectors in communication with a mobile unit send traffic to the mobile unit. The redundancy in traffic was needed to meet the low-delay requirements of voice data for handoff. This constraint is relaxed in a data network.
In a packet data network, users may tolerate short delays in the data transmission. Since low delay is no longer a constraint on the system, reliability can be more efficiently achieved through re-transmission rather than redundant transmission through all the active sectors all the time in handoff scenario. Thus, in a conventional high rate packet data cellular system, data traffic is typically routed through one sector that maximizes the forward link throughput. To accomplish this routing, the mobile monitors all the active sectors, among which the user selects the best and informs the network of its selection. Such a system exploits the channel dynamics in order to maximize the capacity. The selection of the transmitter to exploit local peaks in the shadowing process is a form of selection diversity. Thus, the selection of the best serving sector is also referred to as site selection transmit diversity (“SSTD”).
FIG. 1 illustrates a typical CDMA data communication system, such as CDMA HDR. Access network 100 contains several access points, of which only access points 110 and 130 are shown. A mobile unit, such as access terminal 114, communicates with an access point, such as access point 110, to connect to access network 100. In general, an access point, such as access point 110, will have several sectors, such as sectors 116, 118, and 120.
Since access terminal 114 generally communicates with one sector at a time, data going to access terminal 114 from access point 110 must be routed to the specific sector with which access terminal 114 is communicating.
However, a problem emerges when an access terminal is constantly switching among sectors. Suppose sector 116 has the strongest forward link signal at one instance such that access terminal 114 selects sector 116 as the current serving sector. In the next instance, sector 132 of access point 130 has the strongest forward link signal. Just moments later, sector 116 again has the strongest forward link. It is possible that rapid switching between the two or more sectors can occur. Each time a switch occurs, data that was going to be sent to access terminal 114 must be sent to the corresponding data queue for that sector. Further, the user cannot receive data before the data queue is ready. Such rapid transitions can create a significant amount of overhead for the network and outage for the user.
A second problem for selecting the best sector is related to the reverse link reliability. On the reverse link, access terminal 114 may send channel state feedback information to the network to assist the network in achieving the highest forward link throughput. In the high data network system, access terminal 114 transmits a data rate control signal (“DRC”) to control the data rate on the forward link. Access terminal 114 also sends an acknowledge signal (“ACK”) to the serving sector when it successfully receives a packet. Access terminal 114 should select a new sector that has a reliable reverse link connection with access terminal 114. Otherwise, DRC and ACK information can be lost, which reduces the throughput of the system. However, access terminal 114 does not readily know the reliability of a reverse link connection. If access terminal 114 selects a sector with an unreliable reverse link, throughput can suffer due to retransmission.
Ideally, access terminal 114 should select a new sector so that its throughput on the forward link is maximized. Firstly, the site selection should avoid fast toggling. Secondly, the site selection should incorporate the impact of the reverse link reliability on forward link throughput. Thus, there is a need in the art for methods and systems for properly selecting the best serving sector in a CDMA data communication system.